Cyano(triphenylsilyl)phosphanide as a Building Block for P,C,N Conjugated Molecules.

The cyano(triphenylsilyl)phosphanide anion was prepared as a sodium salt from 2-phosphaethynolate. The electronic structure of this new cyano(silyl)phosphanide was studied via computational methods and its reactivity investigated using various electrophiles and Lewis acids, demonstrating its P- and N-nucleophilicity. The ambident reactivity is in agreement with computations. The silyl group also shows lability and therefore the cyano(silyl)phosphanide can be considered as a phosphacyanamide synthon, [PCN]2- , and serves as building block for the transfer of a PCN moiety.

Phosphorus analogues of common nitrogen anions are of interest as building blocks. Examples are the development of [PCO]− chemistry in the last few decades and the recent report of a [CP]− transfer reagent. The chemistry of compounds with an NCN unit such as in cyanamides like Ca[N=C=N] produced on large scale, or carbodiimides, or species containing an NCC group is well‐established. In contrast, compounds with a PCN sequence are comparatively little investigated.

Bulky substituents R and R′ are required to allow isolation of 1‐phospha‐3‐azaallenes, R−P=C=N−R′ which include functionalised derivatives with a sterically demanding disilyl, phosphanyl, or boryl substituent at phosphorus. Related phosphaallenes, R−P=C=CR′2, with bulky substituents were likewise characterised.[ , ] Anionic P,C,N derivatives or compounds, which can at least be regarded as synthons for these, are especially rare. The best‐known examples are salts of the dicyanophosphide anion, [P(CN)2]− A (Scheme 1), the phosphorus analogue of dicyanamide, first reported in 1977 by Schmidpeter et al. This anion is stable in combination with 18‐crown‐6 (18‐C‐6) coordinated sodium or potassium cations and can be obtained either by CN abstraction from P(CN)3 or in a disproportionation reaction of tetraphosphorus, P4, using potassium cyanide as a reagent.

Scheme 1

Relevant anions A, C, D, F, I with a central PCN or PCC unit or equivalents (E, G, J) thereof (Dipp=2,6‐iPr2‐C6H3, DippTer=2,6‐Dipp‐ C6H3, [Ti]=(hydrotris(3‐ tert‐butyl‐5‐methylpyrazol‐1‐yl)borate)titanium. Generic formula of silyl, germyl, and stannyl cyano phosphides [P(EPh3)(CN)]− [1]−, [2]−, [3]− reported in this work (counter cation [Na(18‐C‐6)]+).

The synthesis of stable tetraphenylphosphonium salts [Ph4P][P(CN)2] and [Ph4P][As(CN)2] is also possible while ammonium salts of [P(CN)2]− are unstable.

Various adducts of the type (NHC)−PCN (NHC=N‐heterocyclic carbene) (see B in Scheme 1) demonstrate the use of [P(CN)2]− as PCN transfer reagent. Remarkably, [P(CN)2]− is a weak nucleophile but reacts as an electrophile with reagents such as phenyllithium to give [P(Ph)CN)]−. Alkynyl phosphanide C, a related PCC anion (Scheme 1), was prepared by deprotonation of the corresponding alkynyl‐substituted secondary phosphine. 1‐Aza‐3‐phosphaallenide anions like [PCNiPr]− D (Scheme 1) behave as ambident nucleophiles and react via the nucleophilic nitrogen or phosphorus center. Deprotonation of aryl cyanophosphines gives cyanophosphides but only sterically protected species such as I (Scheme 1) are stable while smaller derivatives eliminate cyanide salts.

The phosphorus analogue of the cyanamide dianion, [PCN]2−, was studied theoretically. Recently, compounds were discovered which can be viewed as synthons for [PCN]2−. These are the phosphanyl phosphacyanide salt E and the bis(triphenylstannyl)phosphanyl cyanide F which can be used to synthesize phosphaallenes such as R−P=C=N=BR′2, metal complexes such as [(LAu)3PCN]+, or solutions of the parent phosphacyanamide H2PCN. Other functionalised PCN and AsCN derivatives such as H, G and J, were likewise reported lately.

Here we report the synthesis of the salt [Na(18‐C‐6)] [P(SiPh3)(CN)], [Na(18‐C‐6)][1], and show that this push–pull‐substituted phosphide [1]− is a precursor to a number of compounds which contain a 1‐phospha‐3‐aza‐allene unit.

The best results were achieved by adding a freshly prepared mixture of Na[N(SiMe3)2] and Ph3SiCl dissolved in toluene/DME to a concentrated solution of Na(OCP) in toluene/DME at −20 °C, followed by warming to room temperature and stirring the reaction mixture for two days (Scheme 2, see Supporting Information for details). Then 18‐C‐6 is added, which allows to isolate the salt [Na(18‐C‐6)][P(SiPh3)CN], [Na(18‐C‐6)][1], as crystalline powder (64 % yield). The 31P{1H}‐NMR spectrum ([D8]THF) shows a signal at δ(31P)=−283 ppm (1 J P,Si=68 Hz) for the anion [P(SiPh3)(CN)]−, [1]−.

Scheme 2

Preparation of [Na(18‐crown‐6)]+ salts of [1]−, [2]−, and [3]− from Na(OCP)(dioxane)2.14. Relevant resonance forms according to NRT analysis for anion [1]− (DFT, M06‐2X/Def2‐SVP). Relative energies in kcal mol−1 (DFT, M06‐2X/Def2‐SVP) for the 1,3‐silyl group migration from [1]− to [1′]−.

During the synthesis, another species, 4 is detected by 31P{1H}‐NMR at δ(31P)=−295 ppm after a few hours but this signal disappears during the progress of the reaction (see below).

The germyl and stannyl phosphaketene derivatives Ph3E−P=C=O (E=Ge, Sn) were likewise reacted with Na[N(SiMe3)2] to give solutions of the corresponding anions [Na(18‐C‐6)][P(EPh3)CN], E=Ge: [Na(18‐C‐6)][2] [δ(31P)=−278 ppm], and E=Sn: [Na(18‐C‐6)][3] [δ(31P)=−315 ppm], respectively. But these salts are unstable and decompose forming insoluble precipitates, which in the case of [3]− contained NaCN. Similar observations have been reported with anions of the type [Ar‐PCN]−. Nevertheless single crystals of [Na(18‐C‐6)][2] and [Na(18‐C‐6)][3] were grown at low temperatures by layering a solution of [Na(18‐C‐6)][N(SiMe3)2] with a solution of Ph3E‐PCO. The structures of all compounds [Na(18‐C‐6)][1-3] were determined by X‐ray diffraction (XRD) experiments (see the SI for details). Selected bond lengths and angles are given in Table 1. Exemplary, the structure of [Na(18‐C‐6)][1] in a single crystal grown from 1,2‐difluorobenzene (DFB) is shown in Figure 1 showing a dimeric aggregate in which one oxygen center in each 18‐C‐6 acts as a bridge between two sodium cations.

Table 1

Crystallographic and spectroscopic data for anions [1]−–[3]−.

	[1]−	[2]−	[3]−
δ(31P) in ppm	−283	−278	−315
δ(13C) in ppm	139.5	n.d.	n.d.
P‐E in Å	2.2059(4)	2.2784(5)	2.4428(6)
P‐C in Å	1.761(1)	1.763(2)	1.758(2)
C‐N in Å	1.161(1)	1.160(3)	1.165(3)
E‐P‐C [°]	95.50(4)	92.29(6)	93.15(8)

Figure 1

Structure of [Na(18‐c‐6)][1] (hydrogens omitted for clarity) showing the formation of dimers via sodium cation bridging μ2‐O oxygen centers in the 18‐crown‐6 units.

The P−C (1.760±0.003 Å) and C−N (1.162±0.003 Å) bond lengths in all compounds lie within a narrow range and are very similar to those observed in D and F. In contrast to [iPrN=C=P]− D (P−C 1.603(3) Å; C−N 1.248(5) Å], in which the substituent is bound to the nitrogen center, the P−C bonds in the [Ph3E−P−CN]− anions are significantly longer while the CN bond is significantly shorter. This indicates that of the two resonance structures [Ph3E−P−−C≡N] (I) ↔ [Ph3E−P=C=N−] (II), the first is the main contributor to the electronic ground state. This is confirmed by DFT calculations (M06‐2X/Def2‐SVP, Scheme 2) and natural resonance theory (NRT), which show that the phosphide resonance structure has a weight of 76 % while the 1‐aza‐3‐phosphaallenide structure has a weight of only 20 %.

The calculation also indicates that the P‐silylated isomer [Ph3Si−P−C≡N]− [1]− is slightly more stable than the N‐silylated isomer [P=C=N−SiPh3]− [1′]− (+2.5 kcal mol−1). A large energy barrier of 47.2 kcal mol−1 would prevent an equilibrium via a 1,3‐silyl shift (see the Supporting information for details). The transition state (TS) for this process shows a bent η3‐bound PCN unit attached to the SiPh3 group. Related structures have been observed in a lanthanide complex of the [SCP]− anion and in a titanium complex of [Ad‐NCP]−.

The reaction of Na(18‐crown‐6)[1] with one equivalent of chlorotriphenylsilane in toluene or C6D6 produced instant precipitation of colorless NaCl and gave a single product 4 (δ (31P)=−295 ppm; Table 2). 29Si satellites (1 J P,Si=47 Hz) indicate a P−Si bond. This species is identical to the one observed as intermediate in the preparation of [1]− (see above) and suggests that the formation of Ph3Si−P=C=O from Na(OCP) and Ph3SiCl is slower than the formation of 4 from [Ph3Si−P−CN]− and Ph3SiCl.

Table 2

Spectroscopic data for adducts 4–6.

	4	5	6
δ(31P) in ppm	−295	−292	−328
δ(29Si) NSi in ppm	−23	−23	−28
δ(13C) PCN in ppm	170	172	169
1 J P,C in Hz	89	95	101

Single crystals containing 4 and one 18‐crown‐6 molecule could be obtained at low temperature (−30 °C). An XRD experiment allowed us to assign the structure of 4 unambiguously as a 1‐phospha‐3‐azaallene Ph3Si−P=C=N−SiPh3. The structure is shown in Figure 2 A. Compound 4 decomposes in solution into an insoluble dark material over a few days (likely due to an auto‐redox process).

Figure 2

Structures of 4 (A), Na(18‐crown‐6)(DFB)[7] (B), Na(18‐crown‐6)[8] (C), and Na(18‐crown‐6)(THF)[9] (D) (hydrogens, THF (D), Na (C & D) and 18‐crown‐6 (A, C, D) omitted for clarity). Selected bond distances [Å] and angles [°]. A) 4: P1–Si1 2.2572(5), P1–C1 1.6843(15), N1–C1 1.182(2), Si2–N1 1.7456(13); C1‐P1‐Si1 91.87(5), N1‐C1‐P1 176.57(13), C1‐N1‐Si2 165.74(13). B) Na(18‐crown‐6)(DFB)[7]: P1–Si1 2.2224(8), P1–C1 1.734(2), N1–C1 1.162(3), N1–B1 1.573(3); Si1‐P1‐C1 100.19(7), P1‐C1‐N1 174.22(18), C1‐N1‐B1 173.0(2). C) Na(18‐crown‐6)[8]: P1–C1 1.7722(14), N1–C1 1.1550(18), Si1–N2 1.7622(11), N2–C2 1.4016(15), O1–C2 1.2380(16), P1–C2 1.8243(13); N1‐C1‐P1 172.80(13), C1‐P1‐C2 94.70(6), O1‐C2‐P1 127.86(10), O1‐C2‐N2 117.48(11). D) Na(18‐crown‐6)(THF)[9] P1–C2 1.809(2), P1–C1 1.770(3), C1–N1 1.146(3), C2–N3 1.411(3); C2‐N2 1.295(3), C1‐P1‐C2 99.51(10), N1‐C1‐P1 168.6(2).

The P−C bond distance in 4 (1.68 Å) is in the typical range of P=C double bonds and significantly shorter than the one in [1]− (1.76 Å). On the other hand, the C−N bond distance is only slightly longer in the neutral allene 4 (1.18 Å vs. 1.16 Å in [1]−) and still close to a C≡N triple bond distance. Both, the C‐N‐Si angle (166°) and P‐C‐N angle (177°) are close to linearity while the Si‐P‐C angle remains small (92°). When the reaction is performed in a solvent with a polarity higher than toluene such as tetrahydrofuran (THF) or DFB, a broad resonance at δ(31P)≈−293 ppm indicates a dynamic process. Upon cooling a DFB solution to 243 K, the 31P{1H}‐NMR signal of 4, including the 29Si satellites, as well as a broad resonance at δ(31P)≈−281 ppm typical for [1]− is observed, indicating an equilibrium [Ph3Si−P=C=N−SiPh3] (4) ⇄ [Ph3Si−P=C=N]− [Ph3Si(solv)]+ (see [Ph3Si][1], Scheme 3). This prompted us to inspect the relative energies of 4 and its constitutional isomers (Ph3Si)2N−C≡P 4′ and (Ph3Si)2P−C≡N 4′′ by DFT. With respect to the phospha‐aza‐allene 4 (0.0 kcal mol−1), the amino phosphaalkyne 4′ is 3.0 kcal mol−1, and the bis(silyl) cyano phosphane 4′′ 5.1 kcal mol−1 less stable. Calculations of the 31P‐NMR chemical shifts give a very good agreement for 4 [δ(31P)exp=−300 ppm; δ(31P)calc=−305 ppm] while the calculated shifts for 4′ [δ(31P)calc=−98 ppm] and 4′′ [δ(31P)calc=−216 ppm] do not correspond to any experimentally observed resonances, and hence, make the presence of these species unlikely.

Scheme 3

Reactions of Na(18‐crown‐6)[1] with various main‐group element electrophiles and Lewis acids at room temperature.

Upon mixing Na(18‐crown‐6)[1] with Ph3GeCl in C6D6, three species were initially detected by 31P{1H}‐NMR. One major signal at δ(31P)=−292 ppm and two signals of low intensity at δ(31P)=−195 ppm and δ(31P)=−295 ppm. The latter disappears overnight. The 29Si{1H}‐NMR spectrum shows one singlet at δ(29Si)=−23 ppm and one doublet of low intensity at 1 ppm (1 J P,Si=48 Hz). After 16 hours at room temperature, the resonance at δ(31P)=−292 ppm is the most intense one and the 13C{1H}‐NMR spectrum of the reaction mixture exhibits a doublet at δ(13C)=172 ppm (1 J P,C=95 Hz; Table 2), which is very similar to the 13C‐NMR resonance observed in 4 (δ(13C)=170 ppm, 1 J P,C=89 Hz). DFT was used to calculate the 31P‐NMR chemical shifts and relative energies of possible isomers Ph3Ge−P=C=N−SiPh3 5 [δ(31P)calc=−326 ppm; 0.0 kcal mol−1], Ph3Si−P=C=N−GePh3 5′ [δ(31P)calc=−214 ppm; +9.0 kcal mol−1], (Ph3Si)(Ph3Ge)P−C≡N 5′′ [δ(31P)calc=−151 ppm; +4.9 kcal mol−1], and (Ph3Si)(Ph3Ge)N−C≡P 5′′′ [δ(31P)calc=−303 ppm; +6.8 kcal mol−1]. A comparison between the experimental and calculated data, the similarity between the NMR data of 4 and the new compound 5 (see Table S2), and specifically the absence of a 1 J P,Si coupling, allows assigning the major product 5 in this reaction to Ph3Ge−P=C=N−SiPh3 where the Ph3Si and Ph3Ge groups have mutually changed places with respect to the expected product Ph3Si−P=C=N−GePh3 5′. Very likely, the latter is indeed formed initially as a kinetic product with δ(31P)=−295 ppm, but rearranges to the thermodynamically most stable isomer. As for 4, this could be possible via a dissociative pathway as outlined on top of Scheme 3, which is moreover supported by the fact that solutions of 5 in THF or DFB again show broad 31P‐NMR resonances. The minor compound with δ(31P)=−195 ppm does not find any fit among the calculated structures, but its chemical shift agrees with the reported one of [P(CN)2]− [δ(31P)=−193 ppm ].

The reaction of [Na(18‐C‐6)][1] with one equivalent Ph3SnCl in C6D6 gives a very similar result. One product 6 is observed and the NMR data (δ(31P)=−328 ppm (J P,Sn=795 Hz and 833 Hz); δ(29Si)=−28 ppm; δ(13C)=169 ppm (1 J P,C=101 Hz); Table 2) unambiguously allow to assign to this the structure Ph3Sn−P=C=N−SiPh3. Note that both 5 and 6 decompose slowly in solution, likely in a redox process similar to thermal decomposition of 4, and could not be obtained in pure nor crystalline form.

The reaction between Na(18‐crown‐6)[1] with triphenylborane in DFB gave rise to a major 31P{1H}‐NMR signal at δ(31P)=−289 ppm (1 J P,Si=55 Hz) and a broad signal at δ(31P)=−283 ppm of very low intensity which could not be assigned. The major compound is [Na(18‐crown‐6)(DFB)][Ph3Si−P=C=N−BPh3], Na(18‐C‐6)[7]. Layering the reaction solution with hexane afforded crystals of this product. In the anion [7]−, the boryl group binds to the nitrogen atom. As expected, the P−C bond (1.73 Å) is shorter than in the anion [1]−, but significantly longer than in the neutral phospha‐aza‐allene 4. The sodium atom has a weak contact to the phosphorus center (Na‐P distance 3.07 Å) and is furthermore loosely attached to one fluorine center of DFB (Na‐F distance 2.61 Å). The central P=C=N−B unit is again almost linear (P1‐C1‐N1 174°, C1‐N1‐B1 173°).

Na(18‐crown‐6)[1] reacts also with heteroallenes such as mesityl isocyanate or 1,3‐di‐p‐tolylcarbodiimide as electrophiles to give Na(18‐C‐6)[8] and Na(18‐C‐6)[9] as products, respectively. 31P{1H}‐NMR of Na(18‐C‐6)[8] shows a broad resonance at δ(31P)=−101 ppm at room temperature in THF or DFB as solvent. Crystallisation from DFB gave single crystals suitable for XRD. The structure of Na(18‐C‐6)[8] is displayed in Figure 2 C. Anion [8]− is a phospha‐cyanourea derivative with a short C−N bond (1.15 Å), an elongated P=C double bond (1.77 Å), and an almost linear PCN unit (173°). The 13C‐NMR resonance of the C2 carbon in [8]− is observed at δ(13C)=138 ppm (1 J P,C=105 Hz) which is very similar to the data found in [1]−. Note that the reaction between Mes‐N=C=O and the neutral phosphaazaallene, Ph3Si−P=C=N−SiPh3 (4) gives a product with the same NMR data as seen for Na(18‐C‐6)[8] indicating that [8]− with a solvated [Ph3Si]+ as counter cation is formed as a product. The broadening of the 31P‐NMR resonance seen in solutions of [8]− is likely due to a silatropic rearrangement whereby the N‐bound Ph3Si group in [Ar(Ph3Si)N−(C=O)−P−CN]− is transferred to the oxygen center of the C=O group to give [ArN=C(OSiPh3)−P−CN]−. Indeed the calculated energy difference between both isomers is only 1.4 kcal mol−1 and the broad coalescence signal splits into two broad signals at 175 K (Supporting Information Figure S35).

A similar adduct, Na(18‐crown‐6)[9] {δ(31P)=−120 ppm, δ(13C)=134 ppm, 1 J P,C=105 Hz} is obtained from the reaction between Na(18‐crown‐6)[1] and bis(p‐tolyl)carbodiimide. The salt Na(18‐crown‐6)[9] can be viewed as a phosphorus analogue of dicyandiamide [or 2‐cyanoguanidine, H2N(C=NH)(NHCN)[ , ]]. The structure of Na(18‐crown‐6)[9] is shown in Figure 2 D. The bond distances within the PCN unit closely resemble those of [8]−, with a P1‐C1 distance of 1.77 Å and a C1‐N1 distance of 1.15 Å. The only significant structural difference between [8]− and [9]− is the P‐C‐N angle which is closer in [9]− (168°) likely because of steric repulsion between the p‐tolyl substituent at N2 and the bulky [Na(18‐crown‐6)]+ cation which coordinates to the cyano group (Na‐N1 distance 2.35 Å).

In summary, a convenient synthesis to a cyano(silyl)phosphide, [P(SiPh3)(CN)]−, [1]−, has been developed which in contrast to its heavier congeners, [P(EPh3)(CN)]− (E=Ge, Sn), is stable as salt with [Na(18‐C‐6)]+ as counter cation. Counterintuitively, the silyl group binds to the phosphorus center in the thermodynamically most stable isomer although the energy difference to the constitutional (expected) isomer [P=C=NSiPh3]− is very small. In reactions with R3ECl, 1‐phospha‐3‐aza‐allenes R3E‐P=C=N−SiPh3 are obtained, even though calculations indicate that the major contributor to the electronic ground of [1]− is the phosphide and not allenide form. Other possible isomers such as (Ph3Si)(R3E)P−C≡N or (Ph3Si)(R3E)N−C≡P are thermodynamically slightly less stable and their formation from R3E−P=C=N−SiPh3 via a 1,3‐R3E shift may be severely kinetically hindered. Reactions with electrophilic organic allenes such as isocyanates or carbodiimides allow to access new phospha‐cyanurea or phospha‐cyanguanidine derivatives. All salts represent new functional groups in organophosphorus chemistry which may allow to prepare P,C,N conjugated molecules and materials.

Conflict of interest

The authors declare no conflict of interest.

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